The most commonly used lead-free solders at the present time are Sn—Ag—Cu based solder alloys. Sn—Ag—Cu based lead-free solders have excellent heat cycle properties, and they have good wettability among lead-free solders. In addition, like conventional Sn—Pb based solders, they can be formed into any shape. However, as is the case with Sn—Ag based lead-free solders, Sn—Ag—Cu based lead-free solders develop a rough surface (minute irregularities) due to non-uniform solidification, resulting in the surface with almost no luster, and they sometimes develop solidification defects called shrinkage cavities which look like cracks. These phenomena are caused by the growth of dendriform crystals (dendrites) caused by the formation of hypereutectic Sn—Ag.
With Sn—Ag based lead-free solders or Sn—Ag—Cu based lead-free solders, when molten solder solidifies, dendrites of Sn precipitate first as primary crystals, and then a Sn—Ag or a Sn—Ag—Cu eutectic phase solidifies. Due to volumetric shrinkage which takes place at this time, minute irregultarities or shrinkage cavities develop in the solder surface. When observed in detail, it can be seen that the shrinkage cavities develop along dendrites.
Thus, shrinkage cavities are formed by solidification cracking which occurs along the grain boundaries of dendrites, and they extend no further than the surface of solder. Therefore, shrinkage cavities are thought that they do not induce cracking by becoming the starting points of cracks and hence do not impair the reliability of soldering. However, it is difficult to distinguish shrinkage cavities from cracks by external observation, so depending on the size of shrinkage cavities, it is not possible to eliminate the possibility of their affecting the reliability of soldering. Therefore, as stated below, the occurrence of shrinkage cavities is now seen as a problem.
As the soldering interspace (spacing between portions to be soldered) becomes finer and finer, it becomes increasingly difficult for an inspector to distinguish between shrinkage cavities and cracks by visual observation of solder. In some cases, it is not possible to make a determination without using extremely expensive equipment such as an electron microscope. However, there is a limit on the size of a sample which can be observed with an electron microscope, and the time required for observation is at least several hundred times that required for usual equipment for inspecting external appearance. Therefore, it is impossible to use an electron microscope to inspect every solder portions.
In the case of soldering by the reflow method using a solder paste, as the soldering interspace becomes smaller, the printed thickness of solder paste may become as small as around 50 μm. With a printed thickness of this level, the thickness of soldered joints (solder fillets) which are formed after reflow soldering becomes around 25 μm, and the amount of solder in each soldered joint becomes small. If a shrinkage cavity having a depth of around 10 μm develops in such a thin soldered joint, the thickness of the soldered joint locally becomes as thin as around 10 μm, and there is a possibility of the thinness of the soldered joint having an adverse effect on the reliability of soldering.
When performing mounting on a printed circuit board using a solder paste, which is the form in which solder is typically supplied in the reflow soldering method, the solder paste is applied to prescribed locations of a printed circuit board by printing and then subjected to reflow to form soldering portions on the printed circuit board. The printed circuit board is then inspected by an optical method such as image recognition to ascertain the soldered condition of the soldering portions. Optical inspection equipment which is currently sold for lead-free solder is designed to be able to recognize solder having a low luster. However, minute irregularities and shrinkage cavities which develop in the surface of a Sn—Ag based lead-free solder or a Sn—Ag—Cu based lead-free solder are not caused solely by the solder composition, but they vary in extent in accordance with external factors such as the soldering conditions and the cooling conditions. As a result, it is difficult to determine an average value of brightness intensity, and inspection errors can easily take place.
The present applicant disclosed a lead-free solder ball which has a composition of Sn—4.0 to 6.0 mass % Ag—1.0 to 2.0 mass % Cu and which has good surface properties and suppresses the occurrence of minute irregularities and shrinkage cavities in the solder surface (below-identified Patent Document 1). In addition, the applicant disclosed a method of preventing the occurrence of shrinkage cavities in soldered joints during soldering by the flow soldering method using a Sn—Ag—Cu based lead-free solder alloy by carrying out soldering while controlling a solder bath so as to have an Ag concentration of greater than 3.8 mass % to at most 4.2 mass % and a Cu concentration of 0.8-1.2 mass % (below-identified Patent Document 2).
These patent documents are intended to reduce the occurrence of minute irregularities and shrinkage cavities in the solder surface by adding a larger amount of Ag and Cu compared to a Sn-3.5Ag-0.7Cu and Sn-3.0Ag-0.5Cu composition which are currently widely used as lead-free solder alloys. Namely, by increasing the content of Ag and Cu and correspondingly decreasing the Sn content, the formation of Sn dendrites is decreased and the occurrence of surface irregularities and shrinkage cavities is suppressed. However, increasing the content of Ag which is expensive causes material costs to become high. In addition, increasing the content of Ag and Cu causes the melting temperature (particularly the liquidus temperature) of solder to become high and makes it difficult to use the solder for reflow soldering.    Patent Document 1: JP 2003-1481 A    Patent Document 2: JP 2005-186160 A